In the microfabrication using a resist composition, with an increase in the integration degree of an integrated circuit, formation of an ultrafine pattern is required. With such a trend, an exposure wavelength tends to become shorter, for example, from g-line to i-line to KrF laser or ArF laser, and further in recent years, development of a lithography technique using an electron beam, an X-ray or EUV light in place of an excimer laser beam proceeds.
However, from the standpoint of comprehensive performance as a resist, it is extremely difficult to find an adequate combination of a resin, a photo-acid generator, a basic compound, an additive, a solvent and the like to be used and in particular, the recent demand of forming an ultrafine pattern (for example, having a line width of 50 nm or less) with high performance is actually not yet sufficiently fulfilled. Further, the microfabrication using a resist composition is not only directly used in the production of integrated circuit but also is applied to the production of a so-called imprint mold structure in recent years. Thus, in order to sufficiently respond to these applications, it becomes an important problem to form an ultrafine pattern (for example, having a line width of 50 nm or less) in a state where high sensitivity and high resolution are satisfied at the same time.
In the electron beam (EB) lithography, it is known that the influence of electron scattering in the resist film, that is, forward scattering, is reduced by increasing the accelerating voltage of EB. Therefore, in recent years, there is a tendency to increase the accelerating voltage of EB. However, when the accelerating voltage of EB is increased, the influence of scattering of electrons reflected on a resist substrate, that is, back scattering increases instead of reducing the influence of forward scattering. The influence of the back scattering is particularly large in the case of forming an isolated line pattern having a large exposure area. Therefore, the increase of accelerating voltage of EB may cause, for example, reduction in the resolution of an isolated line pattern.
In particular, in the case of patterning a photomask blank used for semiconductor exposure, since a light-shielding film containing a heavy atom, for example, chromium, molybdenum or tantalum is present as an underlayer of the resist film, the influence of the back scattering attributable to refection from the resist underlayer is more prominent in comparison with the case of coating a resist on a silicon wafer. Therefore, the case of forming an isolated line pattern on a photomask blank is susceptible to the influence of the back scattering and the resolution is highly likely to decrease.
A chemical amplification positive resist is, in general, a photosensitive composition containing a compound (photo-acid generator) which generates a strong acid upon light irradiation and a compound in which a hydrophobic acid-unstable group is decomposed upon a catalyst action of the acid generated, thereby converting the compound to an alkali-soluble substance. The photosensitive composition may further contain a basic compound in order to inhibit dark reaction in the unexposed area. The presence of the basic compound makes it possible to deactivate an acid generated due to the influence of electrons scattered through a neutralization reaction to suppress decomposition of the acid-unstable group in the unexposed area. However, in the case of using an amine as the basic compound, since the amine evaporates from the film in each step of resist film formation, exposure and after heating, suppression of the decomposition reaction of the acid-unstable group in the unexposed area is insufficient.
Thus, methods of using various ionic compounds as the basic compound have been investigated. For example, in Patent Document 1, a betaine type compound is proposed as the basic compound. However, the structure acting as the base in the betaine type compound is a carboxylic acid anion group and it has slight acidity after the neutralization of the acid generated so that the suppression effect on the decomposition reaction in the unexposed area is insufficient.
Further, the microfabrication using a resist composition is not only directly used in the production of integrated circuit but also is applied to the production of a so-called imprint mold structure in recent years (see, for example, Patent Documents 2 and 3, and Non-Patent Document 1). Therefore, particularly in the case of forming a pattern using an X-ray, a soft X-ray or an electron beam as an exposure light source, it is also an important problem to satisfy high resolution and the resist performance, for example, roughness performance at the same time, and the solution of the problems has been desired.